1. Field of the Invention
This invention relates to the design of an aerodynamic drag reduction geometry for maximizing the drag reducing capability of geometric shapes mounted or configured on the aft section of land-based vehicles.
2. Discussion of the Prior Art
It is well known that attempts to streamline the aft or rear section of land-based vehicles have the potential to reduce aerodynamic drag and improve fuel efficiency. Various concepts have been proposed and disclosed in the public domain literature including patents and aerodynamic texts. However, none of these designs describes an optimized shape for reducing aerodynamic drag. This is due to the attempts by authors to apply classical aircraft-based aerodynamic theory and intuition to land-based vehicle applications. Aircraft present an ideal model to apply aft section aerodynamic geometry due to their predictable forebody streamlining, their zero degree yaw flow stream condition during flight, and their generally cylindrically shaped fuselages. In addition, aircraft aerodynamic drag reduction systems are generally intended to function as the aircraft flies in three-dimensional space free of the ground.
To predict fluid flow around a body such as an airfoil, classical aerodynamic theory assumes that the airfoil is remote from any stationary surface. Land-based vehicles such as autos and trucks are an exception to classical theory because of the proximity of the vehicle to the ground. The problem is complicated by the generation of turbulence by the wheels, axles, and other structure under the vehicles. Because predictable flow dynamics of a vehicle proximal to the ground are not predicted accurately by theory, optimizing land-based vehicle aft section geometry requires empirical analysis and testing.
Land-based vehicles are often subjected to yaw flow-stream conditions due to crosswinds. Classical aerodynamic theory and intuition once again do not accurately predict yaw-induced turbulent flow and related drag. Combining the effects of the proximity of the vehicle to the ground with yaw flow conditions results in a complex nonpredictable turbulent flow condition that can only be described as chaotic. This nonpredictability of flow dynamics is characteristic of land-based vehicular aerodynamics and requires sophisticated empirical testing in wind tunnels or drag tanks to begin to understand and design an optimized aft section aerodynamic geometry.
In the case of a vehicle such as a semi-tractor trailer, the typical rear or aft geometry is a sharp-edged rectangular box. The forebody, including the tractor and trailer front section and sides, is usually partially `rounded` followed with a rectangular box-like geometry. Such geometry is noncharacteristic of basic aircraft design principles and thus presents a problem for classical theory. Such geometry requires empirical testing and analysis to begin to optimize aft section geometry to reduce drag. In the patent literature, many authors propose aft section geometries that are typical of classical approaches. Many proposed geometric shapes follow the classic approach of a conical or ogive shape that tapers to an apex, but in order to achieve maximum drag reduction these devices are usually too long to be practical. Other authors offer designs that do not specify the dimensions of the aft section aerodynamic geometry, thus failing to teach and omitting the most important criteria for a functional drag reducing device. Kerian, U.S. Pat. No. 4,601,508; Elliot, Sr. U.S. Pat. No. 4,702,509; Sutphen, U.S. Pat. No. 4,741,569; Andrus, U.S. Pat. No. 5,236,347; and Flemming, U.S. Pat. No. 5,240,306 describe aerodynamic drag reducers for land-based vehicles that are neither optimized nor found to be practical.